1. Field of the Invention
The present invention relates to a method of and to a device for detecting micro-scratches in a semiconductor wafer.
2. Description of the Related Art
In the manufacturing of semiconductor devices, chemical and mechanical planarization (CMP) is a global planarization (GP) process for polishing a wafer. In CMP, slurry is sprayed onto the wafer and the wafer is polished with the slurry using a polyurethane polishing pad. The slurry contains silica particles as an abrasive. Some of the silica particles have a diameter equal to or greater than 1 xcexcm which greatly exceeds the average diameter of the silica particles making up the slurry. These large particles apply abnormal stresses to the wafer during polishing. Portions of the wafer to which abnormal stresses have been applied fracture to relieve the stress.
FIG. 1 is a magnified view of the surface of a scratched wafer, and FIG. 2 is a cross-sectional view of a scratched wafer.
As shown in FIG. 1, scratches shaped like human eyebrows are formed on a wafer by CMP. Generally, the width of such a scratch is 0.3 to 3.0 xcexcm, the length thereof is 3 to 30 xcexcm, and the depth thereof is about 200 to 2000 xc3x85. Although micro-scratches on the surface of a wafer can not be easily observed after CMP, the micro-scratches are enlarged during etching because weaker portions of the wafer bearing the micro-scratches are etched more heavily than the other portions of the wafer. The micro-scratches once so enlarged can be easily observed.
It is known that even when large silica particles account for only about 0.1% of the slurry, a large number of scratches are produced. However, it is difficult to remove large silica particles from the slurry, and to measure the amount of the large silica particles.
Moreover, the polyurethane polishing pad is porous and also produces micro-scratches in the wafer. The surface of the polishing pad has a roughness of several xcexcm in virtue of the pores existing at the surface of the polishing pad. Slurry collects in the pores open at the surface of the polishing pad. When polishing is started in this state, stress is applied to the polishing pad by the wafer. Thus, polishing is performed by part of the slurry that exists at the surface of the wafer and in the pores of the pad. During this process, large silica particles lodged in the pores of the polishing pad scratch the surface of the wafer.
Conventional methods for detecting scratches include a method of irradiating monochrome laser light onto a surface and detecting the sizes of scratches in the surface using the light scattered from the surface, a method using scattered light from dies and detecting the scratches by a die-to-die signal processing, and a method of obtaining a highly-magnified video image of a surface using white light as an optical source and detecting scratches in the surface using the signal difference between pixels of the video camera.
However, these conventional methods of detecting scratches or defects in a wafer cannot effectively detect micro-scratches since they do not possess a high enough degree of resolution or an accurate enough image recognition ability.
An object of the present invention is to provide a method by which defects such as micro-scratches in a wafer can be effectively detected, and to provide a device for doing the same. That is, an object of the present invention is to provide a method of and a device for detecting micro-scratches, which method and device provide a high recognition rate of wafer defects and which allow the presence of such defects to be promptly confirmed.
To achieve this object, the present invention provides a method of detecting micro-scratches, in which the surface of a wafer is scanned with light from a light source while the light remains incident upon the surface of the wafer at a predetermined angle, the light reflected by normal portions of the surface ( i.e., at an angle of reflection substantially identical to that of the angle of incidence) propagates to an optical detector, the light received by the optical detector is used to generate an electrical signal indicative of the intensity of the received light, values are assigned to the electric signal in correspondence with the intensity of the light represented thereby, and whether defects are formed at the surface of the wafer is determined by comparing the values assigned to the electrical signal to each other.
Preferably, the light reflected by the surface of the wafer is divided into s polarized light and p polarized light, and the electrical signal is obtained from the s polarized light and/or the p polarized light.
Furthermore, the method also preferably takes measures to lengthen the optical path along which light travels from the light source to the wafer and/or from the surface of the wafer to the optical detector, thereby allowing flexibility in design for providing an appropriate angle of incidence. Such measures include bending the light by reflection and/or refraction. Moreover, the method includes a step of adjustably controlling the angle of incidence.
It is also preferable that the measures taken to lengthen the optical path cause the light to be confined in a first space located between the light source and the wafer and in a second discrete space located between the surface of the wafer and the optical detector. In this case, the light is reflected several times in each of the discrete spaces.
The present invention also provides a device for detecting micro-scratches, including a wafer stage, an optical system which produces light directed onto the wafer, a scanning mechanism which moves the optical system and the stage relative to one another so that the light is scanned across the wafer, a signal detection system which receives the light reflected by the wafer and converts the reflected light into an electrical signal, and a signal analysis system which analyzes the electrical signal. The optical system includes a light source, and light produced by the light source is directed onto the wafer surface at a predetermined angle. The signal detection system which receives the reflected light generates an electrical signal indicative of the intensity thereof. The signal analysis system compares values of the electrical signal with reference to different portions of the wafer surface which are scanned by the light, and this comparison yields a determination of whether defects have been formed in the surface of the wafer.
Furthermore, the signal detection system may include a polarization element which divides the reflected light into s polarized light and p polarized light, and polarized light detectors for converting the polarized light into electrical signals. The signal analysis system preferably determines from these electrical signals whether micro-scratches are present in the surface of the wafer.
A grazing optical system for lengthening the optical path is provided between the optical system and the wafer and between the wafer and the signal detection system. The grazing optical system has first and second mirrors disposed parallel to each other and perpendicular to the planar surface of the wafer. The light source projects light onto one of the mirrors at a predetermined angle of incidence, and light is reflected from one of the mirrors onto the surface of the wafer.
A third mirror may be provided for reflecting light from the light source onto one of the first and second mirrors. The inclination of the reflective surface of the third mirror is adjustable so that the third mirror controls the direction of the incident light.
Still further, a fourth mirror may be interposed between the first and second mirrors. In this case, the fourth and first mirrors coact and the second and fourth mirrors coact to confine the light to first and second discrete spaces, respectively. That is, the path along which light travels from the light source to the wafer, and the path along which the light reflected by the wafer travels to the signal detection system are separate from one another.